The present invention relates to imaging and, more particularly, to detection of and correction for object motion in an imaging system where successive views from different positions are taken, each view representing a two-dimensional projection of the three-dimensional object.
Although the present invention may be employed in other types of imaging systems such as, for example, in X-ray computed tomography imaging, for concreteness of description the following disclosure is directed toward the invention in the environment of an emission tomographic system.
One type of emission tomographic system of interest is the single photon emission computed tomography (SPECT) system in which a low-level gamma ray emitter is injected into the body of a patient. The gamma ray emitter is conventionally of a type which preferentially travels to an organ whose image is to be produced. A large-area planar gamma ray detector detects gamma rays emitted from the body normal to its plane. This information is digitally stored as an image in an M by N array of elements called pixels. The values of M and N are conventionally equal to each other, and are commonly 64 or 128 units, or pixels, across the two dimensions of the image.
A SPECT system employs a plurality of views each taken by positioning a detector parallel to, and at an angle about a fixed axis. The angle is incremented in equal steps between views. The images thus captured are computer-processed to construct pictures of transaxial slices of the body. Computer processing utilizes portions of each succeeding view to reconstruct each transaxial slice. If the body being imaged changes position between successive views, the data from one view does not properly align with data from other views. As a consequence, images reconstructed from the data may be blurred, or contain artifacts of distortion in the imaging process which do not represent actual characteristics of the object being imaged.
In order to minimize the radiation dosage to which the patient is exposed, the injected gamma ray materials are of relatively low radioactivity. As a consequence, each view requires a substantial time such as, for example, about 40 seconds, to produce. If a total of 64 views on a 360-degree arc is desired, angularly spaced apart by about 5.6 degrees, then the entire imaging process takes about 40 minutes to complete. Blurring or distortion can take place when the body being imaged moves a distance on the order of one image pixel. A typical image pixel is about one-half centimeter square. Keeping a human body still to within one-half centimeter for 40 minutes is difficult, if not impossible. Thus, body motion and resultant image degradation are common.
The existence of body motion may be detected from the recorded data in a procedure, called cine mode, in which the entire set of views from the scan is displayed, one after the other, to create a simulated motion picture from which a human operator may observe whether an unacceptable amount of body motion took place during data collection. This technique is basically a quality control method for determining whether the collected data is usable. If the data is unusable, the alternatives are either to discount suitably the image data collected during image interpretation or to repeat the data collection. This technique does not provide means for correcting the data to remove motion-derived errors. In addition, the determination by the operator is at least partly subjective and thus open to operator error.
A further way of detecting body motion, called sinogram, is an image created by displaying the collected data which will later be used to construct a transaxial slice. The human operator is relied on to observe artifacts of body motion by visually detecting certain distortions in the sinogram image. As with cine mode, this is primarily a quality-control technique and does not permit correction of motion-derived errors. It similarly suffers from the need for subjective judgements by the operator.
A further technique such as disclosed, for example, in an article by J. S. Fleming entitled "A Technique for Motion Correction in Dynamic Scintigraphy", in the European Journal of Nuclear Medicine (1984) volume 9, pages 397-402, employs gamma ray emitting point sources applied to the body being imaged. The point sources are imaged along with the remainder of the body. Detected motion of the point sources may be used in a quality-control procedure and may provide sufficient data to apply manual correction factors to some of the affected data. This technique suffers the drawback that the presence of the point sources increases the radiation dosage to which the body is subjected. In addition, during full-circle data collection, the point sources are sometimes located at positions where they are blocked from the detector array by the body. Passage through the body may strongly attenuate the gamma radiation, thus degrading the ability to locate these point sources.
An automated technique for motion correction in an angiography system is disclosed in an article by Alain Venot and V. LeClerk entitled "Automated Correction of Patient Motion and Gray Values Prior to Subtraction in Digitized Angiography", in the IEEE Transactions of Medical Imaging, Volume M1-3, No. 4, December 1984, pages 179-186. This technique maximizes a deterministic sign change criterion with respect to two translational shifts and one constant value. When the shifts and constant value are such that the resultant noise-free image is close to zero, any noise in the image produces a signal shift, either positive or negative with respect to zero. At zero signal (noise only) the probability of the signal changing from plus to minus or vice versa is 0.5. This produces the maximum number of sign changes. Non-optimum values of the criterion place the resulting image farther from zero and superimposed noise has a reduced probability of producing a sign change. Thus, maximizing the sign change best compensates for patient motion.
A further article by Manbir Singh et al, entitled "A Digital Technique for Accurate Change Detection in Nuclear Medical Images--With Application to Myocardial Perfusion Studies Using Thallium-201", in IEEE Transactions on Nuclear Science, Volume NS-26, No. 251, February 1979, Pages 565-575, attempts registration of separate images taken at intervals of, for example, a week, wherein one of the images is taken under stress of exercise, and the other is taken unstressed. The method described in this paper requires user interaction, unlike the automatic techniques of the present invention.